Most cables insulated by sulfur hexafluoride (SF.sub.6) that have been made in the past are single-phase cables (see in particular the Journal Electra, No. 94, 1984). A three-phase line is thus built up from three or four single-phase cables.
The main advantage is as follows: in the event of a line fault, the fault is necessarily a single-phase fault and applies to one only of the conductors, and it cannot propagate to the other conductors. There is therefore only one conductor that needs to be repaired, which is quicker, simpler, and less expensive than repairing a three-phase cable. When a line has four conductors, the fourth conductor is in reserve, and the grid can be brought back into service even more quickly since it suffices to switch the faulty phase over to the fourth conductor. The damaged phase can then be repaired in reasonable time without particular urgency.
The disadvantages of lines comprising a plurality of single-phase cables are the following:
higher resistive losses; PA1 higher cost; and PA1 uses a larger area of ground.